U.S. patent application number 13/518575 was filed with the patent office on 2012-10-11 for welding method and superconducting accelerator.
Invention is credited to Fumiaki Inoue, Katsuya Sennyu, Shuho Tsubota.
Application Number | 20120256563 13/518575 |
Document ID | / |
Family ID | 44482882 |
Filed Date | 2012-10-11 |
United States Patent
Application |
20120256563 |
Kind Code |
A1 |
Tsubota; Shuho ; et
al. |
October 11, 2012 |
WELDING METHOD AND SUPERCONDUCTING ACCELERATOR
Abstract
Provided is a welding method of welding a cylindrical stiffening
member to an outer circumference of a superconducting accelerator
tube body using a laser beam in a process of manufacturing a
superconducting accelerator tube. The laser beam is configured such
that a distribution profile of energy density on an irradiated face
to which the laser beam is irradiated is a Gaussian distribution
profile having a peak section, and the energy density of the peak
section is 5.8.times.10.sup.5 W/cm.sup.2 or more.
Inventors: |
Tsubota; Shuho; (Tokyo,
JP) ; Sennyu; Katsuya; (Tokyo, JP) ; Inoue;
Fumiaki; (Tokyo, JP) |
Family ID: |
44482882 |
Appl. No.: |
13/518575 |
Filed: |
February 10, 2011 |
PCT Filed: |
February 10, 2011 |
PCT NO: |
PCT/JP2011/052875 |
371 Date: |
June 22, 2012 |
Current U.S.
Class: |
315/501 ;
219/121.64 |
Current CPC
Class: |
H05H 7/22 20130101; B23K
26/0823 20130101; B23K 26/1462 20151001; H05H 7/20 20130101; B23K
26/0626 20130101 |
Class at
Publication: |
315/501 ;
219/121.64 |
International
Class: |
B23K 26/20 20060101
B23K026/20; H05H 7/00 20060101 H05H007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 17, 2010 |
JP |
2010-032515 |
Claims
1. A welding method of welding a cylindrical stiffening member to
an outer circumference of a superconducting accelerator tube body
using a laser beam in a process of manufacturing a superconducting
accelerator tube, wherein the laser beam is configured such that a
distribution profile of energy density on an irradiated face to
which the laser beam is irradiated is a Gaussian distribution
profile having a peak section, and the energy density of the peak
section is 5.8.times.10.sup.5 W/cm.sup.2 or more.
2. The welding method according to claim 1, wherein the energy
density of an outer circumference of a region in which 50% of the
total energy in the distribution profile of energy density is
included centered on the peak section is less than or equal to 75%
of the energy density of the peak section.
3. The welding method according to claim 1, wherein the
superconducting accelerator tube body and the stiffening member are
formed of niobium.
4. The welding method according to claim 1, wherein the irradiated
face, front and rear of the irradiated face in a welding direction,
and a reverse face of the irradiated face inside the
superconducting accelerator tube body are supplied with an inert
gas.
5. The welding method according to claim 4, wherein the inert gas
is supplied from a center nozzle installed so as to surround the
laser beam, a front nozzle installed in front of the center nozzle
in the welding direction, a rear nozzle installed in the rear of
the center nozzle in the welding direction, and a reverse face-side
nozzle installed toward the reverse face of the irradiated face
inside the superconducting accelerator tube body.
6. The welding method according to claim 1, wherein the welding
method includes supplying an inert gas between the stiffening
member and the superconducting accelerator tube body.
7. The welding method according to claim 6, wherein, between the
stiffening member and the superconducting accelerator tube body, a
partition plate is installed to partition a space in a
circumferential direction, and the stiffening member includes a
supply port through which the inert gas is supplied to an inside
thereof on one side of circumferential direction of the partition
plate, and a discharge port that discharges gas of the inside
thereof on the other side of circumferential direction of the
partition plate.
8. The welding method according to claim 1, wherein the
superconducting accelerator tube body and the stiffening member are
installed so that central axes thereof are in a horizontal
direction, the laser beam is irradiated to an upper side above the
central axes of the superconducting accelerator tube body and the
stiffening member, and the superconducting accelerator tube body
and the stiffening member are rotated about the central axes in a
direction opposite to a direction directed toward the laser beam
from an upper end of the superconducting accelerator tube.
9. A superconducting accelerator having the superconducting
accelerator tube manufactured by the welding method according to
claim 1.
10. A superconducting accelerator having the superconducting
accelerator tube manufactured by the welding method according to
claim 2.
11. A superconducting accelerator having the superconducting
accelerator tube manufactured by the welding method according to
claim 3.
12. A superconducting accelerator having the superconducting
accelerator tube manufactured by the welding method according to
claim 4.
13. A superconducting accelerator having the superconducting
accelerator tube manufactured by the welding method according to
claim 5.
14. A superconducting accelerator having the superconducting
accelerator tube manufactured by the welding method according to
claim 6.
15. A superconducting accelerator having the superconducting
accelerator tube manufactured by the welding method according to
claim 7.
16. A superconducting accelerator having the superconducting
accelerator tube manufactured by the welding method according to
claim 8.
Description
TECHNICAL FIELD
[0001] The present invention relates to a welding method used in
the process of manufacturing a superconducting accelerator tube and
a superconducting accelerator having the superconducting
accelerator tube.
[0002] This application claims priority to and the benefits of
Japanese Patent Application No. 2010-032515 filed on Feb. 17, 2010,
the disclosure of which is incorporated herein by reference.
BACKGROUND ART
[0003] In the related art, in the process of manufacturing a
superconducting accelerator tube, electron-beam welding is carried
out to weld members. The electron-beam welding is accompanied with
many additional tasks because vacuum drawing is essential, and
positioning requires more time than welding in the air.
[0004] On the other hand, laser welding enables welding procedures
in the air, and is applied to the process of manufacturing the
superconducting accelerator tube, so that efficient manufacturing
can be expected.
[0005] Patent Document 1 discloses a manufacturing method in which,
on a butt weld zone of a straight pipe for a superconducting
cavity, a groove has a stepped shape, and non-piercing welding is
conducted from an inside and then non-piercing welding is conducted
from an outside by a laser beam.
PRIOR ART DOCUMENT
Patent Document
[0006] Patent Document 1: Japanese Patent No. 3959198
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0007] Since partial penetration welding is performed so that a
welded part does not penetrate a base metal, a blowhole (air
bubble) is easily generated from the welded part. To prevent the
blowhole from being generated, defocusing a beam so that an aspect
ratio (depth/width) of a penetration geometry is reduced has been
considered. However, when a metal such as niobium (Nb, melting
point of about 2500.degree. C.) whose melting point is higher than
those of other metals is used as a base metal, and when the beam is
defocused, it is difficult to melt the metal and thus to perform
welding.
[0008] Further, when welding is performed using a high peak beam in
order to melt a high melting-point metal such as niobium, the width
of a bead is narrowed, and the blowhole is more easily generated.
In the partial penetration welding, since the curvature of a bead
bottom is reduced, there is a risk of the bead penetrating the base
metal or of a convexity being formed on a reverse face of a welding
face. Accordingly, a quality of the superconducting accelerator
tube cannot be stably secured.
[0009] Furthermore, in the laser welding, despite the possibility
of the welding procedures in the air, when niobium is used, which
is particularly susceptible to oxidation, it is difficult to
prevent the oxidation to perform the welding procedures of high
quality.
[0010] The present invention has been made keeping in mind the
above problems occurring in the related art, and an object of the
present invention is to provide a welding method of preventing a
blowhole from being generated and allowing a high quality of
partial penetration welding without a bead penetrating a base metal
and without a convexity being formed on a reverse face of a welding
face, and a superconducting accelerator having a superconducting
accelerator tube produced by the welding method.
Means for Solving the Problems
[0011] To achieve the object, the present invention provides a
welding method, in which, when a cylindrical stiffening member is
welded to an outer circumference of a superconducting accelerator
tube body using a laser beam in a process of manufacturing a
superconducting accelerator tube, the laser beam is configured such
that a distribution profile of energy density on an irradiated face
to which the laser beam is irradiated is a Gaussian distribution
profile having a peak section, and the energy density of the peak
section is 5.8.times.10.sup.5 W/cm.sup.2 or more.
[0012] In the present invention, since the energy density of the
peak section is 5.8.times.10.sup.5 W/cm.sup.2 or more, even when
the superconducting accelerator tube body and the stiffening member
are formed of a metal material having a high melting point, they
can be sufficiently melted.
[0013] The laser beam is configured such that the distribution
profile of energy density is the Gaussian distribution profile.
Thereby, a weld zone between the superconducting accelerator tube
body and the stiffening member is configured such that a
circumferential surface of a keyhole has a smooth shape and a bead
having a small aspect ratio is formed. This causes air bubbles in
the molten metal to be easily floated and discharged, and prevents
the molten metal from flowing into and collapsing the keyhole and
entangling the air bubbles. As a result, it is possible to inhibit
the generation of the blowholes.
[0014] Further, the laser beam is configured such that the
distribution profile of energy density is the Gaussian distribution
profile. Thereby, partial penetration welding can be performed
without a bead penetrating the superconducting accelerator tube
body and without a convexity being formed inside the
superconducting accelerator tube body.
[0015] Since the metal can be melted at the peak section, and since
the energy of an outer edge section, which has a lower energy
density than the peak section, can also be applied to the melting
of the metal, an absorption characteristic of energy can be
improved.
[0016] In the welding method of the present invention, the energy
density of an outer circumference of a region in which 50% of the
total energy in the distribution profile of energy density is
included centered on the peak section may be less than or equal to
75% of the energy density of the peak section.
[0017] In this case, the distribution of energy density from the
peak section toward the outer edge section becomes smooth, and the
absorption characteristic of energy on the outer edge section can
be improved.
[0018] Further, in the welding method of the present invention, the
superconducting accelerator tube body and the stiffening member may
be formed of niobium.
[0019] In this case, performance of the formed superconducting
accelerator tube and a superconducting accelerator having the
superconducting accelerator tube can be improved.
[0020] Further, in the welding method of the present invention, an
inert gas may be supplied to the irradiated face, the front and
rear of the irradiated face in a welding direction, and a reverse
face of the irradiated face inside the superconducting accelerator
tube body.
[0021] In this case, since the irradiated face, the front and rear
of the irradiated face, and the reverse face of the irradiated face
inside the superconducting accelerator tube body can be under an
inert gas atmosphere, the superconducting accelerator tube body and
the stiffening member can be prevented from being oxidized.
Further, even when the superconducting accelerator tube body and
the stiffening member are made of a metal having a high oxidation
tendency, their oxidation can be prevented.
[0022] Further, in the welding method of the present invention, the
inert gas may be supplied from a center nozzle installed so as to
surround the laser beam, a front nozzle installed in front of the
center nozzle in the welding direction, a rear nozzle installed in
the rear of the center nozzle in the welding direction, and a
reverse face-side nozzle installed toward the reverse face of the
irradiated face inside the superconducting accelerator tube
body.
[0023] In this case, the inert gas can be stably supplied to the
irradiated face of the laser beam, the front and rear of the
irradiated face in the welding direction, and the reverse face of
the irradiated face inside the superconducting accelerator tube
body.
[0024] Further, in the welding method of the present invention, the
inert gas may be supplied between the stiffening member and the
superconducting accelerator tube body.
[0025] In this case, the inside of the stiffening member and the
superconducting accelerator tube body can be prevented from being
oxidized.
[0026] Further, in the welding method of the present invention, a
partition plate may be installed between the stiffening member and
the superconducting accelerator tube body so as to partition a
space in a circumferential direction, and the stiffening member may
include a supply port through which the inert gas is supplied to an
inside thereof on one side of the circumferential direction of the
partition plate, and a discharge port that discharges gas of the
inside thereof on the other side of the circumferential direction
of the partition plate.
[0027] In this case, the inert gas supplied from the supply port to
the inside of the stiffening member moves in the space between the
stiffening member and the superconducting accelerator tube body in
the circumferential direction, and is discharged from the discharge
port. As such, the space between the stiffening member and the
superconducting accelerator tube body can be under an inert gas
atmosphere.
[0028] Further, in the welding method of the present invention, the
superconducting accelerator tube body and the stiffening member may
be installed so that central axes thereof are in a horizontal
direction. The laser beam may be irradiated to an upper side above
the central axes of the superconducting accelerator tube body and
the stiffening member. The superconducting accelerator tube body
and the stiffening member may be rotated about the central axes in
a direction opposite to a direction directed toward the laser beam
from an upper end of the superconducting accelerator tube.
[0029] In this case, the metal melted by irradiation of the laser
beam moves toward the upper side due to the rotation of the
superconducting accelerator tube body and the stiffening member,
and is solidified. Thus, the metal does not flow toward the
irradiated face to which the laser beam is irradiated. As such, the
welding can be efficiently performed.
[0030] Further, a superconducting accelerator of the present
invention provides the superconducting accelerator tube
manufactured by any one of the welding methods described above.
[0031] In the present invention, since the superconducting
accelerator includes the superconducting accelerator tube
manufactured by any one of the welding methods described above, the
quality of the superconducting accelerator can be stabilized.
Effects of the Invention
[0032] According to the present invention, the metal material
forming the superconducting accelerator tube body and the
stiffening member can be melted by the peak section of the laser
beam. On the weld zone between the superconducting accelerator tube
body and the stiffening member, the circumferential surface of the
keyhole has a smooth shape, and the bead having a small aspect
ratio is formed. This suppresses the generation of the blowholes,
and allows the partial penetration welding to be carried out
without the beads penetrating the body of the superconducting
accelerator tube body or without the convexity being formed on the
reverse face of the welding face of the superconducting accelerator
tube body. As a result, the superconducting accelerator tube body
and the stiffening member can be efficiently welded, and the
manufactured superconducting accelerator tube and superconducting
accelerator can be stabilized in quality.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1(a) shows an example of a superconducting accelerator
tube according to a first embodiment of the present invention, and
FIG. 1(b) is a cross-sectional view taken along line A-A of FIG.
1(a).
[0034] FIG. 2(a) shows a shape of a defocusing beam, FIG. 2(b) is
an enlarged view of an irradiated face of the defocusing beam of
FIG. 2(a), and FIG. 2(c) is an enlarged view of an irradiated face
of a just focusing beam.
[0035] FIG. 3(a) is a three-dimensional view showing an energy
distribution profile of the irradiated face of the defocusing beam
according to the first embodiment, FIG. 3(b) is a cross-sectional
view taken in an irradiating direction including the peak section
of FIG. 3(a), and FIG. 3(c) is an energy distribution view of the
irradiated face.
[0036] FIG. 4(a) is a three-dimensional view showing an energy
distribution profile of the irradiated face of the just focusing
beam, FIG. 4(b) is a cross-sectional view taken in an irradiating
direction including the peak section of FIG. 4(a), and FIG. 4(c) is
an energy distribution view of the irradiated face.
[0037] FIG. 5(a) shows a shape of weld penetration and a keyhole
when welding is performed using the defocusing beam, FIG. 5(b)
shows a shape of weld penetration and a keyhole when welding is
performed using the just focusing beam, FIG. 5(c) is a
cross-sectional view taken along line B-B of FIG. 5(d) and
explaining a state of sidewalls of the keyhole when the welding is
performed using the just focusing beam, and FIG. 5(d) is a
cross-sectional view taken along line C-C of FIG. 5(c).
[0038] FIG. 6(a) is a three-dimensional view showing an energy
distribution profile of the irradiated face of the defocusing beam
according to another embodiment, FIG. 6(b) is a cross-sectional
view taken in an irradiating direction including the peak section
of FIG. 6(a), and FIG. 6(c) is an energy distribution view of the
irradiated face.
[0039] FIG. 7(a) is a three-dimensional view showing an energy
distribution profile of the irradiated face of the defocusing beam
according to yet another embodiment, FIG. 7(b) is a cross-sectional
view taken in an irradiating direction including the peak section
of FIG. 7(a), and FIG. 7(c) is an energy distribution view of the
irradiated face.
[0040] FIG. 8 shows comparison of welded states using defocusing
beams having different average outputs.
[0041] FIGS. 9(a) and 9(b) are explanatory views of a welding
method according to a second embodiment.
[0042] FIG. 10 is an explanatory view of a welding method according
to a third embodiment.
EMBODIMENTS OF THE INVENTION
[0043] A welding method according to a first embodiment of the
present invention will be described below with reference to FIGS. 1
to 8.
[0044] First, a superconducting accelerator tube according to a
first embodiment will be described.
[0045] As shown in FIGS. 1(a) and 1(b), the superconducting
accelerator tube 1 includes a tube body (superconducting
accelerator tube body) 3 composed of a plurality of half cells 2
joined by welding, and stiffening rings (stiffening members) 4
stiffening the tube body 3.
[0046] Each half cell 2 is obtained by pressing a planar
superconducting material formed of niobium and the like, in a bowl
shape so as to have an opening in the center thereof.
Minor-diameter-side ends 2a of two of the half cells 2 are joined
to form a dumbbell-shaped member 5 (see FIG. 1(a)). A plurality of
dumbbell-shaped members 5 are joined in an axial direction, thereby
forming the tube body 3.
[0047] The tube body 3 includes concave iris sections 6 and convex
equator sections 7 on an outer circumference thereof, and has a
corrugated shape in an axial cross section (see FIG. 1(a)) and an
annular shape in a radial cross section (see FIG. 1(b)).
[0048] The stiffening rings 4 are cylindrical members that are
formed of a superconducting material such as niobium and the like,
and are installed so as to cover the iris sections 6, and are
intended to stiffen the tube body 3. Each stiffening ring 4 is
formed in a cylindrical shape in which two semi-cylindrical members
are assembled, and is configured so that axial ends 4a thereof are
welded adjacent to each iris section 6.
[0049] Each stiffening ring 4 may be configured so that three or
more members into which a cylinder is divided in a radial direction
are assembled. Further, a gap between the members constituting the
stiffening ring 4 may be provided.
[0050] The superconducting accelerator tube 1 having the
aforementioned configuration is used as a member for a
superconducting accelerator (not shown).
[0051] Next, a method of manufacturing the superconducting
accelerator tube according to the first embodiment will be
described with reference to the drawings.
[0052] First, a planar material of pure niobium is pressed in a
bowl shape so as to have an opening in the center thereof, thereby
forming a half cell 2 as shown in FIGS. 1(a) and 1(b).
Minor-diameter-side ends 2a of two half cells 2 are joined to form
a dumbbell-shaped member 5.
[0053] Then, the dumbbell-shaped member 5 and the stiffening ring 4
are welded.
[0054] In detail, the dumbbell-shaped member 5 and the stiffening
ring 4 are welded by attaching ends 4a of the stiffening ring 4 to
an outer circumferential surface of the dumbbell-shaped member
5.
[0055] In this case, if a weld bead or a convexity due to welding
is formed inside the dumbbell-shaped member, the superconducting
accelerator is reduced in quality. As such, the welding between the
dumbbell-shaped member 5 and the stiffening ring 4 is partial
penetration welding based on a laser beam from the outside, and
furthermore prevents any convexity from being formed inside the
dumbbell-shaped member 5.
[0056] As the welding between the dumbbell-shaped member 5 and the
stiffening ring 4, laser welding based on a beam having the
distribution of energy density as shown in FIG. 3(c) is employed.
Hereinafter, this beam is referred to as a defocusing beam (laser
beam) 11. This defocusing beam 11 will be described below.
[0057] The defocusing beam 11 is irradiated to melt a point of a
weld zone 8 of the dumbbell-shaped member 5 and the stiffening ring
4. The dumbbell-shaped member 5 and the stiffening ring 4 are
rotated about the central axis 9 thereof, and the defocusing beam
11 is irradiated to the entire weld zone 8. Thereby, the
dumbbell-shaped member 5 and the stiffening ring 4 are welded.
[0058] Then, a plurality of dumbbell-shaped members 5 to which the
respective stiffening rings 4 are welded are joined in an axial
direction, and thus the superconducting accelerator tube 1 is
completed.
[0059] Next, the defocusing beam used in the welding method
according to the first embodiment will be described in comparison
with a just focusing beam.
[0060] The defocusing beam 11 is formed into a beam having the
distribution of energy density as shown in FIG. 3 by shifting a
focus of the laser beam as shown in FIGS. 2(a) and 2(b) or by
changing a lens shape. In the present embodiment, the defocusing
beam 11 is formed by shifting the focus. For example, the
defocusing beam 11 may be formed such that, when a lens having a
focal length of 200 mm is used, a defocusing amount is +5 mm, and a
laser beam diameter .PHI. is about 1.67 mm.
[0061] In welding of the related art, a beam having the
distribution of energy density as shown in FIG. 4 is used.
Hereinafter, this beam is referred to as a just focusing beam
(laser beam) 12. The just focusing beam 12 is a beam formed by
adjusting a focus as shown in FIG. 2(c).
[0062] Here, the following description will be made under the
assumptions that the defocusing beam 11 is irradiated and thus a
face perpendicular to the irradiated direction is an irradiated
face 13, and that the just focusing beam 12 is irradiated and thus
a face perpendicular to the irradiated direction is an irradiated
face 14.
[0063] When the defocusing beam 11 represents a distribution
profile of energy density on the irradiated face 13, the central
portion of the irradiated face 13 has a Gaussian distribution
profile (i.e., a bell-shaped three-dimensional profile as shown in
FIG. 3(a), and a bell curve as shown in FIG. 3(b)) in which the
energy density E is high, as shown in FIG. 3. In contrast, when the
just focusing beam 12 represents a distribution profile of energy
density E on the irradiated face 14, the central portion of the
irradiated face 14 has approximately a cylindrical profile in which
the energy density E has a small difference, as shown in FIG.
4(a).
[0064] Any of the laser beams has an average output of 4500 W and a
speed of 2.0 m/min.
[0065] When the defocusing beam 11 is compared with the just
focusing beam 12, the defocusing beam 11 has a greater diameter
than the just focusing beam 12, as shown in FIGS. 3 and 4.
[0066] Further, the defocusing beam 11 has a peak section 11a of
the energy density E at the central section thereof. Similarly, the
just focusing beam 12 has a peak section 12a of the energy density
E at the central section thereof. Between the energy densities of
the peak sections 11 a and 12a (hereinafter, each is referred to as
"peak energy density E.sub.max"), there is no great difference.
However, the energy density E of the defocusing beam 11 is smoothly
reduced from the peak section 11a toward an outer edge section 11b,
whereas the energy density E of the just focusing beam 12 is hardly
reduced from the peak section 12a toward an outer edge section
12b.
[0067] Here, as shown in FIGS. 3(b) and 3(c), the energy density of
an outer circumference 11d of a region 11c in which 50% of the
total energy is included centered on the peak section 11a is set as
energy density E.sub.50. Similarly, as shown in FIGS. 4(b) and
4(c), the energy density of an outer circumference 12d of a region
12c in which 50% of the total energy is included centered on the
peak section 12a is set as energy density E.sub.50. Then, a
fraction of the energy density E.sub.50 with respect to the peak
energy density E.sub.max in the defocusing beam 11 is compared with
that in the just focusing beam 12.
[0068] The defocusing beam 11 according to the present embodiment
has the peak energy density E.sub.max of 6.9.times.10.sup.5
W/cm.sup.2, the energy density E.sub.50 of 5.1.times.10.sup.5
W/cm.sup.2, and the fraction of the energy density E.sub.50 with
respect to the peak energy density E.sub.max of 73.9%.
[0069] Moreover, the fraction of the energy density E.sub.50 with
respect to the peak energy density E.sub.max in the defocusing beam
11 is preferably set to 75% or less.
[0070] Further, the defocusing beam 11 has energy density E.sub.86
of 2.4.times.10.sup.5 W/cm.sup.2. A fraction of the energy density
E.sub.86 with respect to the peak energy density E.sub.max in the
defocusing beam 11 is 34.8%. Note that the energy density E.sub.86
is energy density of an outer circumference of a region in which
86% of the total energy is included centered on the peak section
11a.
[0071] In contrast, the just focusing beam 12 has the peak energy
density E.sub.max of 7.2.times.10.sup.5 W/cm.sup.2, the energy
density E.sub.50 of 6.0.times.10.sup.5 W/cm.sup.2, and the fraction
of the energy density E.sub.50 with respect to the peak energy
density E.sub.max of 83.3%.
[0072] Further, the just focusing beam 12 has energy density
E.sub.86 of 5.1.times.10.sup.5 W/cm.sup.2. A fraction of the energy
density E.sub.86 with respect to the peak energy density E.sub.max
in the just focusing beam 12 is 70.8%.
[0073] In this case, any of the defocusing beam 11 and the just
focusing beam 12 is set such that the peak energy density E.sub.max
has a value of 5.8.times.10.sup.5 W/cm.sup.2 or more, preferably
6.0.times.10.sup.5 W/cm.sup.2 or more. In this manner, the peak
energy density E.sub.max is set to a value greater than
5.8.times.10.sup.5 W/cm.sup.2. Thereby, niobium having a melting
point of about 2500.degree. C. can be melted.
[0074] When the welding is performed using the aforementioned
defocusing beam 11, the peak section 11a evaporates and melts the
metal, and the outer edge section 11b holds a molten state of the
metal but does not further evaporate the metal. As such, a keyhole
15 as shown in FIG. 5(a) is smoothly formed in a wide range.
[0075] In contrast, when the welding is performed using the just
focusing beam 12, the peak section 12a as well as the outer edge
section 12b melts the metal. As such, a deep keyhole 16 as shown in
FIG. 5(b) is formed in a narrow range.
[0076] In the welding based on this just focusing beam 12, as shown
in FIGS. 5(c) and 5(d), metals 17 into which sidewalls 16a of the
keyhole 16 located on lateral and rear sides in a welding direction
(an arrow direction of FIG. 5(d)) are melted are easy to move
toward a bottom 16b of the keyhole 16. With this movement, air
bubbles enter to become blowholes 18.
[0077] Further, in the welding based on this just focusing beam 12,
since the keyhole 16 is deep, there is a risk of a bead penetrating
the metal or a convexity being formed on a reverse face of the
welding face.
[0078] Next, an operation of the welding method according to the
first embodiment will be described using the figures.
[0079] According to the welding method of the first embodiment, the
welding is performed using the defocusing beam 11 in which the
distribution profile of energy density on the irradiated face 13 is
the Gaussian distribution profile and furthermore the fraction of
the energy density E.sub.50 with respect to the peak energy density
E.sub.max is 75% or less. As such, in comparison with the welding
based on the just focusing beam 12 of the same average output, it
is possible to form the beads having a small aspect ratio which can
form the smooth keyhole in a wide range. As a result, the air
bubbles in the metal melted into the weld zone 8 are easily floated
and discharged, and the metals melted into the sidewalls of the
keyhole flows and prevents the air bubbles from being entangled
therein. Thus, it is possible to inhibit the generation of the
blowholes 18.
[0080] Further, since the bead having a small aspect ratio is
formed, and since an evaporation reaction force for forming the
keyhole is weak, the partial penetration welding can be performed
without the keyhole and the beads penetrating the dumbbell-shaped
member 5 or without the convexity being formed on the reverse face
of the welding face of the dumbbell-shaped member 5.
[0081] Further, since the defocusing beam 11 is configured such
that the peak section 11a has the peak energy density E.sub.max of
5.8.times.10.sup.5 W/cm.sup.2 or more, the defocusing beam 11 can
sufficiently melt even a metal having a high melting point such as
niobium.
[0082] In the welding method according to the first embodiment,
since the welding is performed using the defocusing beam 11, the
blowholes 18 of the weld zone 8 can be suppressed, and the partial
penetration welding can be performed without the beads penetrating
the dumbbell-shaped member 5 or without the convexity being formed
on the reverse face of the welding face of the dumbbell-shaped
member 5. As such, the superconducting accelerator tube 1 can be
efficiently manufactured. Further, this stabilizes a quality of the
superconducting accelerator tube 1 and a quality of the
superconducting accelerator having the superconducting accelerator
tube 1.
[0083] Further, since the metal can be melted by the peak section
11a, and since the energy of the outer edge section side having a
lower energy density E than the peak section 11a can also be
applied to the molten metal, an absorption characteristic of energy
can be improved.
[0084] Next, the dumbbell-shaped member 5 formed of niobium and the
stiffening ring 4 are welded using a defocusing beam having a
distribution profile of energy density that is different from that
of the defocusing beam 11 according to the first embodiment, and
then a relationship between the peak energy density E.sub.max and
the fraction of the energy density E.sub.50 with respect to the
peak energy density E.sub.max and a welded state is checked.
[0085] The defocusing beam 19a shown in FIG. 6 has an average
output of 4500 W, a peak energy density E.sub.max of
6.6.times.10.sup.5 W/cm.sup.2, and an energy density E.sub.50 of
3.9.times.10.sup.5 W/cm.sup.2. A fraction of the energy density
E.sub.50 with respect to the peak energy density E.sub.max is
59.1%, and a fraction of the energy density E.sub.86 with respect
to the peak energy density E.sub.max is 22.7%.
[0086] In the welding based on the defocusing beam 19a, the
dumbbell-shaped member 5 and the stiffening ring 4 can be welded,
no bead penetrates the dumbbell-shaped member 5, or no convexity is
formed on the reverse face of the welding face of the
dumbbell-shaped member 5.
[0087] A defocusing beam 19b shown in FIG. 7 has an average output
of 4500 W, a peak energy density E.sub.max of 5.7.times.10.sup.5
W/cm.sup.2, an energy density E.sub.50 of 3.0.times.10.sup.5
W/cm.sup.2, and an energy density E.sub.86 of 1.2.times.10.sup.5
W/cm.sup.2. A fraction of the energy density E.sub.50 with respect
to the peak energy density E.sub.max is 52.6%, and a fraction of
the energy density E.sub.86 with respect to the peak energy density
E.sub.max is 21.1%.
[0088] In the welding based on the defocusing beam 19b, the
dumbbell-shaped member 5 and the stiffening ring 4 are not melted
and cannot be welded. This is attributed to the peak energy density
E.sub.max of 5.7.times.10.sup.5 W/cm.sup.2, and the energy density
E of the peak section being insufficient.
[0089] Next, the dumbbell-shaped member 5 formed of niobium and the
stiffening ring 4 are welded using a defocusing beam having a
distribution profile of energy density that is different from that
of the defocusing beam 11 according to the first embodiment, and
then a relationship between the peak energy density E.sub.max and
the fraction of the energy density E.sub.50 with respect to the
peak energy density E.sub.max and a welded state is checked.
[0090] When welding was performed with specimens HS-10, HS-9, and
HS-8 listed in FIG. 8, HS-10 could be welded, while HS-9 and HS-8
could not be welded.
[0091] It can be seen from this that, even if the defocusing beam
having a different average output has the peak energy density
E.sub.max higher than 5.8.times.10.sup.5 W/cm.sup.2, the partial
penetration welding can be performed under the control of a depth
of penetration.
[0092] Next, another embodiment will be described with reference to
the attached drawings. The same symbols will be used for members or
parts that are the same as or similar to those of the first
embodiment described above, and so description thereof will be
omitted. Thus, the configurations different from those of the first
embodiment will be described.
[0093] As shown in FIGS. 9(a) and 9(b), in a welding method
according to a second embodiment, laser welding is carried out
while an inert gas G is supplied.
[0094] The inert gas G is supplied to an irradiated face 13 of a
defocusing beam 11, the front and rear of the irradiated face 13 in
a welding direction, a reverse face of the irradiated face 13
inside a tube body 3 of a superconducting accelerator tube 1, and a
space 25 between a dumbbell-shaped member 5 and a stiffening ring
4.
[0095] In the present embodiment, the dumbbell-shaped member 5 and
the stiffening ring 4 are welded while being rotated in a direction
of arrow A of FIG. 9. Here, a welding direction is reverse to the
direction of arrow A.
[0096] As shown in FIG. 9(a), inert gas supplying means 21 for
supplying the inert gas G to the irradiated face 13 of the
defocusing beam 11 and the front and rear of the irradiated face 13
in the welding direction includes a center nozzle 22 installed so
as to surround the defocusing beam 11, a front nozzle 23 installed
in front of the center nozzle 22 in the welding direction, and a
rear nozzle 24 installed in the rear of the center nozzle 22 in the
welding direction. The inert gas supplying means 21 is installed
apart from the stiffening ring 4 by a predetermined interval, and
faces 23a and 24a of the front and rear nozzles 23 and 24 which are
opposite to the stiffening ring 4 are formed as curved faces
corresponding to a cylindrical shape of the stiffening ring 4.
[0097] When a welding task is performed, the inert gas G is
supplied from the center nozzle 22, the front nozzle 23, and the
rear nozzle 24 at the same time.
[0098] As shown in FIG. 9(b), the inert gas G is supplied to a
reverse face of the irradiated face 13 inside the tube body 3 of
the superconducting accelerator tube 1 by a reverse face-side
nozzle 29 installed toward the reverse face of the irradiated face
13.
[0099] Alternatively, the welding may be performed in the state in
which the reverse face of the irradiated face 13 inside the tube
body 3 as well as the entire interior of the tube body 3 is under
an inert gas atmosphere.
[0100] Further, as shown in FIG. 9(a), the supply of the inert gas
G to the space 25 between the dumbbell-shaped member 5 and the
stiffening ring 4 is performed as follows.
[0101] A partition plate 26 is installed in the space 25, so as to
partition the space 25 in a circumferential direction. Gas in the
space 25 is unable to pass through the partition plate 26. The
stiffening ring 4 is provided with a supply port 27 through which
the inert gas G is supplied to the space 25 on one side of
circumferential direction to the partition plate 26, and a
discharge port 28 that discharges air of the space 25 on the other
side of circumferential direction to the partition plate 26. The
supply port 27 and the discharge port 28 are installed so as to be
adjacent to each other via the partition plate 26.
[0102] When the inert gas G is supplied from the supply port 27 to
the space 25, the gas in the space 25 is discharged from the
discharge port 28. In this case, since the space 25 is partitioned
with the partition plate 26, the supplied inert gas G moves in the
space 25 in the circumferential direction, and is filled in the
space 25. Then, the inert gas G is discharged from the discharge
port 28.
[0103] In the present embodiment, one partition plate 26 is
provided. However, a plurality of partition plates 26 may be
provided to divide the space 25 between the dumbbell-shaped member
5 and the stiffening ring 4 into a plurality of sub-spaces, and the
supply port 27 and the discharge port 28 may be provided to the
respective sub-spaces.
[0104] The welding method according to the second embodiment
produces a similar effect as in the first embodiment, and stably
supplies the inert gas G to the weld zone 8. Thereby, the
dumbbell-shaped member 5 and the stiffening ring 4 can be prevented
from being oxidized.
[0105] Further, in comparison with a method of performing welding
with an entire chamber under an inert gas atmosphere, in the
welding method according to the second embodiment, the
dumbbell-shaped member 5 and the stiffening ring 4, both of which
are intended for welding, can be easily replaced, and the
dumbbell-shaped member 5 and the stiffening ring 4 can be easily
positioned because no task is performed in the chamber.
[0106] Next, a welding method according to a third embodiment will
be described with reference to the figures.
[0107] As shown in FIG. 10, the welding method according to the
third embodiment is carried out so that a dumbbell-shaped member 5
and a stiffening ring 4 are installed such that axial directions
thereof are in a horizontal direction, and are rotated about the
central axis 9 thereof in a direction of arrow A of FIG. 10. Then,
an irradiated face 13 of a defocusing beam 11 is rotated from an
upper end 4b of the stiffening ring 4 whose axial direction is in
the horizontal direction within a range of 0.degree. to 90.degree.
in a direction opposite to the direction of arrow A of FIG. 10, and
is located at the same height as the central axis 9 or on an upper
side of the central axis 9. Preferably, the irradiated face 13 is
located at an angle between 5.degree. and 45.degree. at which it is
rotated from the upper end 4b of the stiffening ring 4 in the
direction opposite to the direction of arrow A of FIG. 10.
[0108] The welding method according to the third embodiment
produces a similar effect as in the first embodiment. A metal
melted by irradiation of a defocusing beam 11 moves toward the
upper side due to rotation of the dumbbell-shaped member 5 and the
stiffening ring 4, and is solidified. Thus, the metal does not flow
toward the irradiated face 13 to which the defocusing beam 11 is
irradiated. As such, in the welding method according to the third
embodiment, the welding can be efficiently performed.
[0109] While the embodiments of the welding method of the present
invention have been described, the present invention is not limited
to the embodiments, and can be adequately modified without
departing from the scope and spirit thereof.
[0110] For example, in the embodiments described above, the
superconducting accelerator tube 1 and the stiffening rings 4 are
formed of pure niobium. However, they may be formed of a metal
other than pure niobium or a material containing niobium.
[0111] Further, in the second embodiment described above, the inert
gas G is supplied to the irradiated face 13 and the front and rear
of the irradiated face 13 in the welding direction. However, the
inert gas G may be supplied only to the irradiated face 13
depending on a welding speed or a depth of penetration required for
the weld zone 8. Further, the welding may be carried out under an
inert gas atmosphere using another method of supplying the inert
gas G.
INDUSTRIAL APPLICABILITY
[0112] According to the present invention, the metal material
forming the superconducting accelerator tube body and the
stiffening member can be melted by the peak section of the laser
beam. On the weld zone between the superconducting accelerator tube
body and the stiffening member, the circumferential surface of the
keyhole has a smooth shape, and the bead having a small aspect
ratio is formed. This suppresses the generation of the blowholes,
and allows the partial penetration welding to be carried out
without the bead passing through the body of the superconducting
accelerator tube or without the convexity being formed on the
reverse face to the welding face of the body of the superconducting
accelerator tube. As a result, the superconducting accelerator tube
body and the stiffening member can be efficiently welded, and the
manufactured superconducting accelerator tube and superconducting
accelerator can be stabilized in quality.
DESCRIPTION OF REFERENCE NUMERALS
[0113] 1: superconducting accelerator tube
[0114] 3: tube body (superconducting accelerator tube body)
[0115] 4: stiffening ring (stiffening member)
[0116] 6: iris section
[0117] 8: weld zone
[0118] 9: central axis
[0119] 11: defocusing beam (laser beam)
[0120] 11a: peak section
[0121] 11c: region
[0122] 11d: outer circumference
[0123] 13: irradiated face
[0124] 21: inert gas supplying means
[0125] 22: center nozzle
[0126] 23: front nozzle
[0127] 24: rear nozzle
[0128] 25: space
[0129] 26: partition plate
[0130] 27: supply port
[0131] 28: discharge port
[0132] 29: reverse face-side nozzle
[0133] G: inert gas
* * * * *